The in vitro assay for erythroid colony forming units (BFU-E and CFU-E) will be employed as a model system for evaluating the interaction between erythropoietin and erythroid progenitor cells. In particular, we plan to examine the role of monovalent and divalent cations in erythroid cell proliferation using as probes, the chelating agent EGTA, the cardiac glycoside ouabain, the potassium ionophores valinomycin and nigericin and the hormones PTH and calcitonin. Using isokinetic gradient sedimentation, polyester fiber filtration and T lymphocyte specific antiserum, we plan to examine the role of nonerythroid cells in the regulation of erythropoiesis under in vitro conditions. Lectin (PHA, WGA, Con A)-treated mice and mice treated with complete Freund's adjuvant will be employed as model systems to study the influence of lymphocytes on erythropoiesis in vivo. Using the in vivo spleen colony assay, we plan to determine the effect of ouabain, and the potassium and calcium ionophores on the proliferation of mouse hematopoietic stem cells. Finally, purified erythropoietin will be employed to produce antibodies for the development of an immunoaffinity technique for hormone purification and for use in a radioimmunoassay for erythropoietin.